With development of a communication network, a demand for a service bandwidth increases continuously, while an available frequency resource is limited. The CCDP technology is one of main approaches for increasing transmission capacity under the same frequency resource. At the same frequency, the CCDP technology may use two different polarization directions, that is, a horizontal direction and a vertical direction, for transmitting two different channels of signals simultaneously, which may double spectrum utilization.
In a CCDP system, because of multipath fading, scattering of an inhomogeneous layer and the ground, an impact of rain and fog, and a limitation of a manufacturing process of a dual-polarized antenna, a cross-polarized signal causes serious interference to a co-polarized signal. In order to correctly receive a signal, the prior art provides a transmission method where CCDP is used for sending and XPIC is used for receiving. At a sending end: two different channels of service are sent by using the same frequency and different polarization directions; in a transmission channel, because of the limitation of the manufacturing process of the dual-polarized antenna and channel depolarization, the service in two different polarization directions interferes with each other; and at a receiving end: interference cancellation signals mutually sent by the service in the two different polarization directions are received, and a transversal filter of a secondary path extracts a copy of an interference signal to cancel out an interference signal in a primary path, so that service in the primary path is correctly received. As shown in FIG. 1, a CCDP transmission model is shown on the left side in the figure, an XPIC reception model is shown on the right side in the figure, and the middle part shows polarization interference that arises in a transmission channel.
A principle of XPIC is described by taking reception of a V signal by the receiving end as an example. A signal sent out by a polarization splitter (a dual-polarized antenna) is a signal (V+H′) that is mixed with a horizontal interference component, and a signal split by another polarization splitter is an H signal (H+V′) that includes a V signal. The H+V′ signal is sent to a transversal filter, a copy of an interference signal of an H signal in the V signal is formed by using a transversal filter, and this copy is subtracted from a received signal V+Ht to cancel interference of the H signal in the signal V+H′, so that a pure V signal is obtained.
When implementing the transmission method where CCDP is used for sending and XPIC is used for receiving, the inventor finds that a technical solution of the prior art has at least the following problem: When a board of one channel of service at a receiving end fails, a signal cannot be received and an interference cancellation signal cannot be provided for another channel of service, so that both of the two channels of service in the whole system are unavailable.